1. Introduction
This invention relates to light-sensitive compositions. More particularly, this invention relates to aqueous developable light-sensitive compositions useful as high resolution soldermasks in the manufacture of printed circuit boards.
2. Description of the Prior Art
Aqueous developable liquid soldermasks are known. They are typically applied as a wet coating, such as by curtain coating, dried, image-wise exposed to activating radiation, partially cured using heat, developed, such as with an aqueous alkaline solution, and often finally cured. Examples of aqueous developable soldermasks are disclosed in European patent Publication No. 0,255,989 and in Japanese published Patent Disclosures 55-129341 and 60-26943 published Oct. 7, 1980 and Feb. 9, 1985, respectively. Soldermasks disclosed in these publications typically consist of at least one alkali soluble phenolic resin, a compound containing at least two epoxy or vinyl ether groups, generally an epoxy resin, and a photoactive component such as a sulfonium salt or an azide compound capable of initiating crosslinking of the epoxy or vinyl ether compound upon exposure to activating radiation with heating as necessary. The phenolic component, in adequate concentration, enables aqueous development, and the combination of the epoxy or vinyl ether compound and the photoactive component permits cure in exposed areas of the film.
In use, image-wise exposure of a dry film of the above composition to activating radiation initiates a photoreaction, such as the liberation of an acid. The acid catalyzes crosslinking of the epoxy or vinyl ether component of the formulation in imaged areas where the photoreaction occurs. Selective curing results in areas of differential solubility that permit development of the soldermask film.
It has been found that the aqueous developable soldermasks, as disclosed in the referenced publications, fail to yield a high resolution image upon development. It is believed that a cause of poor resolution is the phenolic component of the formulation. Prior to development, the phenolic component is essentially uncured because the liberation of acid by the photoreaction does not catalyze curing of the phenolic component of the soldermask. Consequently, upon contact of an exposed film with developer, selective development is possible between exposed and non-exposed areas because the epoxy or vinyl ether component is cured, but high resolution images are not obtained because the phenolic component is non-selectively dissolved from both exposed and non-exposed portions of the coating alike.
In EPO application publication No. 0,255,989 referenced above, it is disclosed that a thermal curing agent for the phenolic component may be included in the formulation. However, if included, the phenolic of the composition cannot be cured prior to development because curing will take place uniformly in both exposed and non-exposed portions of the film making development difficult or impossible. Therefore, inclusion of a thermal curing agent would not overcome the problem of poor resolution following development.